1. Field of the Invention
The invention relates to proppants useful in oil and gas wells. In particular, it relates to a ceramic proppant material in which proppants of various sizes are included, and will be described with particular reference thereto. It will be appreciated, however, that the invention is also suited to the extraction of other fluids from boreholes, such as water wells.
2. Discussion of the Art
Oil and natural gas are produced from wells having porous and permeable subterranean formations. The porosity of the formation permits the formation to store oil and gas, and the permeability of the formation permits the oil or gas fluid to move through the formation. Sometimes the permeability of the formation holding the gas or oil is insufficient for economic recovery of oil and gas. In other cases, during operation of the well, the permeability of the formation drops to such an extent that further recovery becomes uneconomical. In such circumstances, it is common to fracture the formation and prop the fracture in an open condition by means of a proppant material or propping agent Such fracturing is usually accomplished by hydraulic pressure using a gel-like fluid. The pressure is increased until cracks form in the underground rock. The proppants, which are suspended in this pressurized fluid, are forced into the cracks or fissures. When the hydraulic pressure is reduced, the proppant material functions to prevent the formed fractures from closing again.
A wide variety of proppant materials are used, depending on the geological conditions. Typically, proppants are particulate materials, such as sand, glass beads, or ceramic pellets, which create a porous structure. The oil or gas is able to flow through the interstices between the particles to collection regions, from which it is pumped to the surface. Over time, the pressure of the surrounding rock tends to crush the proppants. The resulting fines from this disintegration tend to migrate and plug the interstitial flow passages in the propped structure. These migratory fines drastically reduce the permeability, lowering the conductivity of the oil or gas. Conductivity is a measure of the ease with which oil or gas can flow through the proppant structure and is important to the productivity of a well. When the conductivity drops below a certain level, the fracturing process is repeated or the well is abandoned.
Ceramic proppants, sometimes called man-made proppants, are favored over natural proppants, such as sand or resin-coated sand, due to their ability to withstand high pressures and temperatures and their resistance to corrosion. Despite being of higher cost than natural materials, the increased crush strength of ceramic renders the ceramic proppants suitable for conditions which arc too severe for other materials, e.g., at rock pressures of above about 350 to 700 kg/cm2 (5000-10,000 psi). As pressure increases with depth, ceramic proppants arc commonly used at depths of about 1500 meters, or more. They are typically formed by combining finely ground material, such as clay, bauxite, or alumina, with water and then mixing in a rotary mixer. Blades in the mixer cause the wet clay to ball up into generally spherical pellets, which upon drying and firing at high temperature are of the general particle size desired. Pellets which fall outside the desired range are returned to the mixer after the drying stage to be reworked.
The crush strength of the proppants is related to the composition and density of the ceramic. Proppants are generally classed in one of three grades: light weight proppants (LWP), intermediate grade proppants (IP), and high strength proppants (HSP). Light weight proppants are suitable for use over a range of closure stress from less than about 1000 psi to about 8000 psi, while intermediate grade proppants are useful up to about 10,000 psi, and high strength proppants can be used at pressures in excess of 12,000 psi. Attempts to improve conductivity have focused on methods of improving crush strength of the proppants. These include the application of coatings, production of stronger spheres, and changes in shape. While measurable improvements in conductivity have been obtained, for example, by applying a resin coating, such improvements have invariably been associated with increases in cost.
Spherical pellets of uniform size have conventionally been considered to be the most effective proppants as they have been thought to maximize conductivity (see, e.g., U.S. Pat. No. 4,623,620). An excess of fines (very small pellets) acts to clog the void space in between the packed spheres, reducing the fluid transport. It is also known that spheres tend to be weaker as the size increases, and are thus more likely to become crushed in situ. In addition to increases in the number of fines, crushing results in a reduction in the width of the crack formed in the fracturing process. Thus, the presence of both small and large particles in a proppant mixture has been thought to be deleterious. Accordingly, the American Petroleum Industry (API) standard, the commonly accepted standard in the industry, requires that the particle size distribution be within fairly narrowly defined limits. For example, particle size ranges are defined according to mesh size designations, such as 40/70, 30/50, 20/40, 16/30, 16/20, and 12/18. The first number in the designation refers to the ASTM U.S. Standard mesh size of the largest (top) sieve and the second number refers to the mesh size of the smallest (bottom) sieve. The API standards require that 90% of the spheres comprising the proppant material be retained between the top and bottom sieve when sieved through the mesh designations for the product.
As a result of the requirements for narrow particle size distributions, only a small proportion of the pellets produced in the forming process are within the predetermined range. The remainder, often as much as 75-80% of the material, must be reground or otherwise treated and reformed in the rotary mixer.
The present invention provides a new and improved proppant material and method of making and use which overcome the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of forming a proppant mixture is provided. The method includes combining a particulate material with a liquid to form a mixture. The mixture is formed into spherical pellets. The pellets are screened and fired to provide a proppant mixture with a size distribution in which: 0-25% wt. % of the total weight of pellets have a diameter of from 1.0-1.18 mm; 20-38 wt. % of the particles have a diameter of from 0.85-1.0 mm; 20-38 wt. % of the pellets have a diameter of from 0.71-0.85 mm; and 15-35 wt. % of the pellets have a diameter of from 0.60-0.71 mm.
In accordance with another aspect of the present invention, a method of forming a proppant mixture is provided. The method includes combining a particulate material with a liquid to form a mixture, forming the mixture into spherical pellets, screening and firing the pellets to provide a proppant mixture comprising pellets having a median diameter of from about 0.6 to 0.85 mm and wherein when the proppant mixture is sieved through a plurality of sieves having ASTM U.S. Standard mesh sizes of 18, 20, 25, 30, and 35: 20-38 wt. % of the pellets are retained by the 20 mesh sieve, 20-38 wt. % of the pellets are retained by the 25 mesh sieve, and 15-35 wt. % of the pellets are retained by the 30 mesh sieve.
In accordance with another aspect of the present invention, a method of forming a proppant mixture is provided. The method includes combining a particulate material with a liquid to form a mixture, forming the mixture into spherical pellets, and screening and firing the pellets to provide a proppant mixture comprising pellets with a size distribution in which: at least 3 wt. % of the pellets have a diameter of from 1 mm-1.18 mm, at least 20 wt. % of the pellets have a diameter of from 0.85-1.0 mm, less than 33 wt. % of the pellets have a diameter of from 0.71-0.85 mm, and at least 10 wt. % of the pellets have a diameter of from 0.60-0.71 mm.
In accordance with another aspect of the present invention, a proppant mixture is provided. The proppant mixture includes ceramic pellets having a sphericity of at least 0.75. At least 3 wt. % of the pellets have a diameter of from 1 mm-1.18 mm, at least 20 wt. % of the pellets have a diameter of from 0.85-1.0 mm, less than 38 wt. % of the pellets have a diameter of from 0.71-0.85 mm, and at least 10 wt. % of the pellets have a diameter of from 0.60-0.71 mm.
In accordance with another aspect of the present invention, a method of propping a geological formation is provided. The method includes combining spherical pellets with a liquid or gel to form a mixture and forcing the mixture under pressure into the geological formation until the pellets arc situated in cracks in the formation, the spherical pellets having a size distribution in which: at least 3 wt. % of the pellets have a diameter of from 1 mm-1.18 mm, at least 20 wt. % of the pellets have a diameter of from 0.85-1.0 mm, less than 38 wt. % of the pellets have a diameter of from 0.71-0.85 mm and at least 10 wt. % of the pellets have a diameter of from 0.60-0.71 mm.
In accordance with another aspect of the present invention, a proppant mixture is provided. The mixture comprises ceramic pellets having a median particle size. When the proppant mixture is sieved through a sequential series of mesh sizes selected from the ASTM U.S. Standard mesh sizes consisting of 12, 14, 16, 18, 20, 25, 30, 35, 40, and 45, the median particle size is in a peak mesh size corresponding to one of the meshes in the series. 0-25% wt. % of the pellets are retained on the sieve having a mesh size which is two sizes preceding the peak mesh in the sequence, 20-38 wt. % of the particles are retained on the sieve having a mesh size immediately preceding the peak mesh in the sequence, 20-38 wt. % of the pellets are retained on the peak mesh, and 15-35 wt. % of the pellets are retained on the next subsequent mesh in the sequence.
An advantage of at least one embodiment of the present invention is that conductivity through a proppant structure is increased.
Another advantage of at least one embodiment of the present invention is that the strength of the proppant mixture is higher, under pressure, than would be conventionally expected from the distribution range.
Another advantage of at least one embodiment of the present invention is that a smaller portion of the green proppant pellets are recycled.
Another advantage of at least one embodiment of the present invention is that the product can be produced at a cost comparable with light weight proppants, but with higher crush resistance and conductivity under pressure.
Another advantage of the present invention is that a user of proppants can replace both IP grade and LWP grade proppants with an Extended PSD intermediate grade proppant mixture, or HSP, IP, and LWP grades with an Extended PSD high strength grade, thus reducing the amount of inventory the user holds.
Another advantage of the present invention is that an increase in conductivity enables increased productivity from a geological formation due to increases in fracture half-lengths. Fracture half-length is a measure of the length of the fracture in a geological feature.
Still further advantages of the present invention will be readily apparent to those skilled in the art, upon a reading of the following disclosure and a review of the accompanying drawings.
xe2x80x9cSphericalxe2x80x9d and related forms, as used herein, is intended to mean an average ratio of minimum diameter to maximum diameter of about 0.75 or greater, or having an average sphericity value of about 0.75 or greater compared to a Krumbein and Sloss chart.
xe2x80x9cSpecific gravityxe2x80x9d is the weight in grams per cubic centimeter (g/cc) of volume, excluding open porosity in determining the volume. The specific gravity value may be determined by liquid (e.g., water or alcohol) displacement or with an air pycnometer.
xe2x80x9cCalcinedxe2x80x9d as used herein, refers to a heating process to which a material has been subjected. Ore materials that have been fully subjected to calcination or a calcining process exhibit very low loss on ignition (LOI) and moisture contents, e.g., about 1-2 percent by weight or less. Uncalcined ore materials such as bauxites and clays can contain from about 10 to about 40 percent by weight volatiles. xe2x80x9cPartially calcinedxe2x80x9d materials typically exhibit total volatiles (LOI plus moisture content) of 5 to 8 percent by weight. Volatiles can include moisture, organics and chemically held water (e.g., water of hydration). Typical calcination temperatures are usually less than 1000xc2x0 C.
xe2x80x9cSinteringxe2x80x9d as used herein refers to a heating process in which the materials are at least partially converted to another form by heating the material to a temperature above that at which such conversion occurs. For bauxites, clays, and the like, conversion typically begins at around 1150xc2x0 C.